After he had joined UNIT and the Third Doctor was apparently in a coma, the Brigadier summoned "Dr Sullivan" to help see him through the process. When Sergeant Benton woke the Doctor by offering him a cup of coffee, the Brigadier cancelled the call for Dr Sullivan's assistance. (TV: Planet of the Spiders)

Immediately after his adventure with the Cybermen, Harry was ejected from the TARDIS and it stranded him in England, 1936. There Harry met the Eighth Doctor, who had no memory of Harry or any of his previous adventures. Harry helped the Eighth Doctor to stop a Dryad from turning the land of Britain on the people that lived upon it. The Fourth Doctor and Sarah picked Harry up shortly after. (PROSE: Wolfsbane)

By 2005, he was back at NATO, having achieved the rank of Commodore. The Brigadier asked him for a favour at the time. (AUDIO: The Wasting)

To commemorate an anniversary date, Sarah Jane had yearly meetings with Harry Sullivan. By 2006, Harry had been missing in action for some time, though Sarah kept coming to the same place they used to meet. She "talked" to him and raised a toast to him wherever he was. (AUDIO: Buried Secrets) Harry was still missing in 2009. (PROSE: The Wedding of Sarah Jane Smith)

Sarah Jane indicated that he had died, by referring to him solely in the past tense, and recalled in 2010 that Harry had worked in developing vaccines that had saved thousands of lives. (TV: Death of the Doctor)

At some point, Harry visited UNIT's Black Archive alongside Sergeant Benton and Sarah Jane to have his record as a companion of the Doctor taken. His memories of the visit were subsequently erased. (TV: The Day of the Doctor)

A version of Harry appeared in a "hellscape" dream the Eleventh Doctor created while the mind parasite Mr Waites fed off the worst thing Doctor could imagine. In the dream, the Doctor worked with Harry at the Department of Commonality. Harry revealed that the protest at the "square" was much larger than reported and that his sister's boyfriend had proof on his camera. After the Doctor didn't enjoy working at mediation and wanted to return to records, he reported to Mr Waites about the camera and the peace officers arrested Harry. (COMIC: John Smith and the Common Men)

Harry was rather old-fashioned and stereotypically English in his attitudes. He often employed slightly archaic language, referring to Sarah affectionately as "old girl" or "old thing" — which slightly annoyed her. (TV: The Sontaran Experiment, PROSE: Doctor Who and the Ark in Space) He was brave, one time saving the Fourth Doctor's life from a landmine on Skaro. (TV: Genesis of the Daleks) He had a "can-do" attitude, adapting well to the many strange situations in which he found himself. He could, however, be quite clumsy and unsubtle. This led the Doctor to declare in a moment of frustration, at the top of his lungs, that "Harry Sullivan is an imbecile!", before passing out from the effects of a rockfall on Voga – caused by Harry. (TV: Revenge of the Cybermen) He flirted a bit with Sarah. Despite an outward annoyance at that attitude, once when thinking him dead, Sarah admitted she found it endearing. (PROSE: Wolfsbane)

Harry has many similarities to Martha Jones: They both worked as medical officers for UNIT whilst working with other organisations, they both had adventures with the Doctor after leaving the TARDIS, both were possessed by enemies of the Doctor, and they both battled Davros, Daleks and Sontarans.

Harry was created as a companion when it was thought that the Fourth Doctor would be played by an older actor who would not be able to handle the physical side of the series. This would necessitate Harry handling physical "two-fisted" action as the plot required. This was not a concern when the 40-year-old Tom Baker was cast in the role, as he was somewhat younger than the previous actors and capable of physical action on his own. Harry was thus deemed redundant and ultimately written out of the show, despite his popularity — a decision that Philip Hinchcliffe later admitted was probably a mistake.

Harry is a member of a small group of companions who did not feature in any TARDIS interior scenes. Although he is seen entering and emerging from the TARDIS several times, he is never seen within the TARDIS itself.

Ian Marter reappeared as Harry in The Android Invasion. Sadly, there were to be no further guest appearances, as the Doctor Who production team had by this time decided to phase out UNIT and sever the Doctor's links with Earth.

It was hoped Ian Marter would reprise the role of Harry for the 20th anniversary story The Five Doctors in 1983, but Marter had to turn down the offer, as he was committed workwise at the time.

Although various spin-off media have depicted Harry living into the 21st century, in real life the actor who portrayed him, Ian Marter, died in October 1986. A reference to the character is made in the television story Invasion of the Bane, when Sarah Jane considers Harry as a possible name for her adopted son, Luke Smith. Perhaps going along with this fact, it has been implied that Harry has indeed already passed away in the present-day continuity of the Doctor Who universe, as in the television story Death of the Doctor, when recounting the current endeavours of some of the Doctor's past companions, Sarah Jane refers to him in past tense.